Spinach (Spinacia oleracea L.) is a flowering vegetable plant belonging to the family Amaranthaceae. It is native to Southwestern and Central Asia, but today spinach is cultivated worldwide, mainly in temperate regions. Spinach plants are cultivated for their highly nutritious leaves. The leaves are extremely rich in antioxidants, a good source of vitamins such as A, B6, C, E, K and folate, and minerals such as calcium, iron, magnesium and potassium.
The edible part of the spinach plant is a compact rosette shape of leaves attached to a short stem. The leaves are produced during the first stage of the plant's life cycle, the vegetative rosette stage. The second stage is the flowering stage, or the bolting stage. When bolting occurs, there is growth of an elongated stalk with flowers growing from within the main stem of the plant. Once the plant has reached the bolting stage, it is no longer possible to harvest marketable leaves. The plant will allocate its resources to flowering instead of leaf production, which will ultimately cause the leaves to wither. Fast bolting is thus a very undesired trait in the production of leafy vegetables such as spinach. Therefore, slower bolting is very much preferred by growers in order to optimize yield.
There are three basic types of spinach, namely savoy, semi-savoy, and smooth. The savoy type spinach have dark green, crinkly and curly leaves. The semi-savoy type spinach is a hybrid variety with slightly crinkled leaves. Smooth type spinach has broad, smooth leaves.
The leaves of a spinach plant are usually sold loose, bunched, in pre-packaged bags, canned or frozen. Each of these products requires its own type of leaves. The smooth and some of the semi-savoy varieties are predominantly used in processed products such as canned or frozen spinach, while the semi-savoy and savoy types are especially used for the fresh market. Smooth leaf varieties are better suited for processing purposes than semi-savoy and savoy varieties, because they are easier to clean and prepare for canning and freezing. Savoy leaf varieties, on the other hand, are preferred in the fresh market because the leaves pack looser than the smooth leaf varieties and are less likely to wilt or turn yellow.
The growth habit of spinach varieties can be classified in three different categories, flat or prostate, semi-erect, and upright or erect. For mechanical harvest an upright or erect habit is preferred in order to reduce soil contamination. Especially for varieties with savoy type leaves this is an important character, because the soil is difficult to remove from curly leaves. Sometimes even plant growth regulators are applied before harvest to cause a more upright leaf growth and reduce the risk of soil contamination. More upright varieties are thus desired by growers to reduce contamination and costs for spraying plant regulators.
Downy mildew is probably the most widespread and potentially destructive global disease of spinach. The causal agent of downy mildew on various Amaranthaceae, including spinach, is regarded as a single species, Peronospora farinosa. In particular, Peronospora farinosa f. sp. spinaciae infects spinach. Initial symptoms of downy mildew consist of dull to bright yellow necrotic lesions that appear on the leaves of infected spinach plants. With time the lesions can enlarge and become tan and dry. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation, resulting in widespread crop damage. In addition to the loss of quality of the leaves due to the lesions, downy mildew can also cause breakdown and rot of the infected leaves if they packaged in bags and cartons.
The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≧80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants. Although some fungicide treatments may be effective, they can be costly and may cause ecological or pollution concerns, and may not be desired by organic farmers.
In recent years various resistance genes have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the fungus. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to P. farinosa overcoming the resistance in these spinach cultivars. The downy mildew races (also called physios, isolates, or strains) that were able to infect resistant spinach cultivars were collected in a differential reference set, which can be used to test spinach cultivars for resistance. The differential set may comprise a series of spinach cultivars (hybrids) that have different resistance patterns to the currently identified pathogenic races.
Currently there are 13 officially recognised races of Peronospora farinosa f. sp. spinaciae, designated races pfs1 to pfs13 (Irish et al. Phytopathol. Vol. 98 number pg. 894-900, 2008; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Denomination of Pfs: 12, a new race of downy mildew in spinach”, Feb. 28, 2011; Plantum NL (Dutch association for breeding, tissue culture, production and trade of see and young plants) press release, “Benoeming van fysio Pfs: 13, een nieuwe fysio van valse meeldauw in spinazie”, Aug. 2, 2011. Recently another new Peronospora isolate has been identified, which has not yet received an official “Pfs” number, but which is assumed to become known as Pfs14 in the near future: UA4410. This isolate is publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, along with the 13 officially recognized Pfs races. See e.g., Correll and Koike, “Race diversity and the biology of spinach downy mildew pathogen”, Mar. 15, 2011 CLRGB Progress Report, and Feng et al, “Characterization of new races and novel strains of the spinach downy mildew pathogen Peronospora farinose f. sp. spinaciae”, 2011 International Spinach Conference Oct. 3-4, 2011, Amsterdam and Enkhuizen, The Netherlands (Abstract in Updated Program Sep. 26, 2011).
Newly identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry. For this reason new resistance genes are very valuable assets. In order to confer a resistance that is as broad as possible, i.e. that confers resistance to as many Pfs races as possible, preferable to all known Pfs races, it is very useful to be able to stack different resistance genes against Peronospora infection in spinach. This is achieved by stacking various resistance genes, which have overlapping resistance patterns. In this way it becomes more difficult for the pathogen to overcome the resistance. Such a combination of different resistance genes on one gene segment is also highly desirable. It is much easier if the resistance genes can be inherited as a single dominant loci, because in that case the resistance pattern that is conferred by the dominant resistance gene cannot segregate in the progeny of the cross, and will always inherit as one single set of resistances to various pathogenic races. One such resistance gene, designated R6, is a single dominantly inherited resistance gene in spinach (See generally EP application No. 11187288).
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.